Many processes and devices make use of moving fluids. Some examples are heating and ventilating systems that make use of moving air, as well as manufacturing processes that rely on the use and flow of various gasses. Typically, for these processes and/or devices, it is important that the flowrate of the fluid be accurately determined. Accordingly, various techniques to determine the flowrate of a fluid have been developed.
One area where flow sensors find particular usefulness is in medical ventilators. Medical ventilator systems have been long used to provide supplemental oxygen support to patients unable to breathe normally on their own accord. Such devices typically comprise a source of pressurized air or oxygen-rich air which is fluidly connected to the patient through a fluid conduit.
Ventilators are used by patients in various health situations. A primary application of ventilators is to relieve breathing problems due to obstructive sleep apnea in which a patient benefits from positive air pressure applied to their breathing passage. Portable home ventilator machines provide a continuous pressure or modulated pressure to help open breathing passages during sleep. Other patients have weak physiological attributes that prevent them from breathing unassisted and require invasive ventilation. Invasive ventilatory support generally requires the patient having either a tracheotomy or endotracheal tube disposed into the trachea of the patient. Such treatment generally occurs in hospitals and is administered in acute care situations or post operative recovery situations.
Manufacturers of medical ventilator equipment require a low pressure drop to insure efficient blower operations. To monitor fluid flow in ventilators, products such as a Honeywell flow channel housing AWM720 can be utilized. An example of the AWM720 device is disclosed in U.S. Pat. No. 6,655,207, entitled “Flow Rate Module and Integrated Flow Restrictor,” which issued to Speldrich et al on Dec. 2, 2003 and is assigned to Honeywell International Inc. U.S. Pat. No. 6,655,207 is incorporated herein by reference in its entirety. The device disclosed in U.S. Pat. No. 6,655,207 generally relates to a flow sensor having a restrictor to create a small pressure drop in parallel with a substrate having a sensing element and flow channel aligned over the sensing element. The sensing element senses at least one property of a fluid. The flow channel across the sensing area is accurately and precisely aligned facilitating reliable, low-cost, and consistent results among multiple flow sensors.
This type of technology, when utilized in the context of a medical ventilator, typically utilizes a low pressure drop restrictor in parallel with a bypass channel to drive a fraction of the flow in the main channel past the mass airflow sensor. This technique measures the mass flow rate of the main channel and volume flow rate can be inferred by knowing the density of the fluid. Although only a small pressure drop in the range of 2 in H2O is added to the system, it is undesirable in medical ventilator applications.
In order to address the needs of flow sensing in medical ventilator applications, it is believed that an improved flow sensor should be implemented. It is believed that the improved fluid velocity sensor disclosed herein can address these and other continuing needs.